Cellular Components and Cytology: Nucleus, Chromosomes, and Microscopy

The Cell Nucleus Structure

Nuclear Envelope

The nuclear envelope is a double membrane structure:

• The outer nuclear membrane is continuous with the Endoplasmic Reticulum (ER) and may have ribosomes.
• The inner nuclear membrane has a fibrous lamina, known as the nuclear lamina.

Nuclear Pore Complex

The nuclear pore is formed by the union of the two membranes and a set of ribonucleoproteins forming the Nuclear Pore Complex (NPC). The NPC regulates the passage of molecules into and out of the nucleus.

Nucleoplasm

The nucleoplasm is the internal environment of the nucleus. It contains a network of fibrillar proteins (the nuclear matrix), which remains fixed and organizes the nucleolus and chromatin.

Nucleolus

The nucleolus is a dense, almost spherical structure where ribosomes are synthesized from rRNA and protein. Each rRNA gene corresponds to a nucleolar organizer. The nucleolus consists of two main areas:

• Fibrillar area
• Granular area

Chromatin Types

Chromatin consists of DNA associated with proteins. There are two main types:

1. Euchromatin: Forms the "string of pearls" structure and contains genes that are actively being expressed (high transcription).
2. Heterochromatin: Formed by the solenoid structure, it contains genes that are inactivated or have low transcription rates. Heterochromatin may be of two types: constitutive or optional.

Chromosomes

The main parts of a chromosome are:

• Centromere
• Telomere
• Arms
• Chromatids
• Satellites
• Secondary constrictions

Chromosome Classification

Chromosomes can be classified based on several criteria:

• By stage of division: They can be anaphasic (depending on the stage of metaphase and division).
• By centromere position: Metacentric, submetacentric, acrocentric, and telocentric.
• By sex determination: Autosomes (22 pairs in humans) and heterochromosomes or sex chromosomes (1 pair).

Interphase Nucleus Characteristics

The interphase nucleus is surrounded by a double membrane or envelope. Within the chromatin and nucleoplasm, one or more nucleoli are found. Depending on the number of nucleoli, the cell may be uninucleate or multinucleated. The nucleus is typically larger in active cells. The nucleoplasm relationship (RNP) is stable for each cell type.

Key Cytological Definitions

Karyotype

A karyotype is the complete set of chromosomes of an organism, ordered by size and type.

Homologous Chromosome (Counterpart Chromosome)

This refers to each chromosome of a pair that a diploid eukaryotic organism possesses and which are paired together during meiosis.

Fundamentals of Cell Theory

The Cell Theory establishes fundamental principles of biology:

• All living things are made of cells.
• The cell is the anatomical, physiological, morphological, and genetic unit of living beings.
• All cells originate from pre-existing cells.

Microscopy Techniques

Microscopes can be classified as optical or electronic.

Optical Microscopes

Optical microscopes have a resolution of 200 nm and are typically formed by a tube and two lenses.

Electron Microscopes

Electron microscopes offer much higher resolution:

• Transmission Electron Microscopes (TEMs): Have a resolution of 2 nm. They are used for ultra-thin cuts (60–100 nm) prepared using ultramicrotomes. The sample must be dehydrated and stained.
• Scanning Electron Microscopes (SEMs): Have a resolution of 10 nm. They bounce electrons off the sample surface to create an image. Unlike TEM, there is no electron flow through the cell, and samples are often prepared by cryofracture.

Principles of Cell Culture

Cell culture involves maintaining living cells and allowing them to reproduce in appropriate conditions. It is used to study various biological processes. Unlike immortal cell lines, primary animal cells have a limited division potential (Hayflick limit). It is often beneficial to work with clones. Sometimes, researchers work with hybrid cells derived from heterokaryons (cells fused from different origins).